Magnetic Resonance Imaging (MRI) involves applying different types of electromagnetic fields and radiofrequency (RF) excitations to a subject. The aim in doing so is to generate spatial RF signals from a specific region of the subject by which an MRI image is generated. The spatial RF signals are proportional to the strength and the homogeneity of the applied magnetic fields. If there is any distortion in the magnetic fields, this causes distortion in the final MRI image which in some cases may lead to a false diagnosis, for example in medical applications where the subject being imaged is a human subject.
Due to the nature of objects being imaged as well as the difficulties attached to engineering magnetic coils, magnetic fields are not perfectly homogeneous. Factors leading to inhomogeneous effects can include internal factors such as production tolerances in the scanner, heating of coils during scanning, vibrations during scanning, or external factors like ferromagnetic material, like iron, that may be in the vicinity of the scanner such as in a surrounding building construction.
MRI scanners have a set of several coils, which typically include a main superconductive coil which produces a powerful main magnetic field (called “B0”) which polarizes an object to be scanned, an RF coil for generating and receiving RF pulses, and magnetic field gradient coils that generate spatial variations in the main magnetic field for spatial encoding of the MRI signal. It is not possible to create a perfectly homogenous main magnetic field (B0) within a bore of the MRI scanner. Therefore, MRI scanners generally include shims, which are active or passive devices which can adjust the homogeneity of the magnetic field. Passive shims consist of metal pieces which are positioned during the installation of the MRI scanner to generate tiny static magnetic fields, and cannot be manipulated during an MRI scan. Active shims consist of gradient coils which generate tiny magnetic fields in three perpendicular directions (X, Y and Z). These fields are generated by calculating offset electrical currents which then pass through the gradient coils. The process of adjusting the active shims is called shimming.
In existing MRI scanners, active shimming of the main magnetic field (B0) is performed once before an MRI scan begins. The fields required to be generated by the active shim coils must be determined by first acquiring a map of the main magnetic field, which includes the offset phase in the B0 filed due to filed inhomogeneity (ΔB0). This map can be obtained using various pulse sequence techniques, such as using a two-TE (where TE refers to the echo time) three-dimensional gradient echo sequence. From the field map, zero, first and even higher order shim parameters are calculated and then the drift in the scanner central frequency (the zero order shim) and the offset in the electrical currents of the shim gradients are estimated from the shim parameters.
Some MRI modalities, such as Functional MRI (fMRI) and Diffusion Tensor Imaging (DTI), which are mainly based on echo planar imaging (EPI) for data acquisition, require scanning a volume of a subject repeatedly. Such repeated scanning could take anything between 6 minutes up to about 40 minutes for some DTI applications. During such a long scanning period, the initial shim prepared by the scanner could be compromised, rendering the final MRI images inaccurate. Temporal changes of the initial prepared shimmed main magnetic field may arise due to factors such as patient respiration, poor shimming of the MRI scanner, or generalized and random patient motion. The changes in the main magnetic field, including drift in a system central frequency (zero order shim) and distortion in the shim magnetic field gradients (first or higher order shims), can cause different types of geometrical distortions in an MRI image such as shift, stretching, contraction, signal loss, image blurring and ineffective RF excitation pulses.
Currently, the mechanism by which various sources of distortion affect the change in the main magnetic field is not well understood, and existing scanners generally do not include an ability to compensate for such changes during the course of scanning. The distortion in the B0 field cannot be addressed with external tracking systems. Several techniques, both hardware and software-based, have been proposed to deal with the distortion in the B0 field, but they come with limitations and drawbacks.
The technology described in this application seeks to address these problems, at least to some extent.